By 'Thermal compound' are you referring to the white zinc oxide grease they
use to attach TO-220's and other heat-sink mounted devices to, or some other
compound?

 Why would someone apply it prior to running the parts through reflow unless
you are suggesting that it's under a BGA or surface mount package that you
cannot inspect from afterwards... The ones I have seen that have heat
sinking in the center of the package where you don't have physical access to
them after installation all required that you actually solder the heat sink
'pad' to a land area on the board directly beneath the chip and provide
'thermal vias' in that land to get the heat out to the planes and mounting.

Heat sink compounds are really there to remove all air pockets and gaps
between the surface of the heatsink and the heat source or device... they
work the best when they are used very sparingly... more compound does not
improve the thermal transfer... in fact if it's very thick it will actually
degrade the thermal transfer and cause the component to heat up more rapidly
due to isolation from the heatsink...

Maybe after you describe  a little more about the example more precisely
someone can help a bit more with the answer to your question. I would tend
to think that thermal compounds would dry out in the heat generated by the
solder reflow process. That white zinc stuff migrates all over the place
too... nasty stuff... I would much prefer to use Sil-pads or something more
akin to that rather than mess with heat sink compounds in general.

Best regards,


Bill Brooks - KG6VVP
PCB Design Engineer, C.I.D.+, C.I.I.
Tel: (760)597-1500 Ext 3772 Fax: (760)597-1510
Datron World Communications, Inc.
_______________________________________
San Diego Chapter of the IPC Designers Council
Communications Officer, Web Manager
http://dcchapters.ipc.org/SanDiego/
http://pcbwizards.com

-----Original Message-----
From: Gregg A. Owens [mailto:[log in to unmask]]
Sent: Tuesday, June 21, 2005 9:38 AM
To: [log in to unmask]
Subject: [TN] Thermal Compound

A student recently asked me if a component is mounted with thermal heat
sinking compound (placed prior to soldering) will not the compound
inhibit the flow of air to the solder destination side of the board. If
so would that not entrap flux? Also the compound will make the solder
destination side un-inspectable? Good questions to which I did not have
a reply. Any thoughts?

Gregg A. Owens
Manufacturing Technology Training Center, Inc.

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